1. Field of the Invention
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-366297, filed on Dec. 18, 2002, the entire contents of which are incorporated herein by reference.
The present invention relates to a color separation unit of a solid-state image sensor which is particularly suited for providing an image without false color, by increasing the efficiency of color separation to increase the sensitivity of the solid-state image sensor.
2. Description of the Related Art
In a color imaging device such as a color video camera and the like, a color separation unit for separating light from a light source into three primary colors mainly adopts one of two methods.
The first method is a three-color separation dichroic prism (three-CCD) method. In the three-CCD method, incident light having been color-separated by the color separation unit, which includes three prisms, an air layer, and a plurality of dichroic filters (for example, a red reflection filter and a blue reflection filter), is applied to the three CCDs. Japanese Unexamined Patent Application Publication No. Hei 5-168023, for example, discloses the three-CCD method (refer to FIG. 2 of the patent document).
In the second method, that is, a single-CCD method, a color separation filter of primary color or additive complementary color is disposed on each light receiving surface of a CCD. Japanese Unexamined Patent Application Publication No. Hei 6-141327, for example, discloses the single-CCD method (refer to page 2 of the patent document).
The three-CCD color separation unit is large and expensive due to the complex structure of an optical system. The single-CCD color separation unit, on the other hand, has the advantage that it is simple, small, and inexpensive. Thus, a video camera, a digital still camera and the like generally use the single-CCD color separation unit.
However, the single-CCD color separation unit has the following problems.
First, the color separation filters disposed in front of the CCD decrease photon utilization efficiency. Therefore, the sensitivity of the CCD decreases.
Second, the different color (red, green or blue) filter is disposed in front of each light receiving surface of the CCD. The color separation filters are arranged in, for example, well-known Bayer Array. Accordingly, the red, green, and blue light receiving surfaces are spatially separate from one another, so that data outputted from each light receiving element corresponding to each light receiving surface has to be interpolated to actualize color. Therefore, there is a problem that false color, which does not exist in reality, appears.
The foregoing problems will be hereinafter described in detail.
FIG. 5 is a sectional view of a color separation unit of a conventional single-CCD image sensor. FIG. 6 is a graph showing the spectral transmittance of a color separation filter used in the color separation unit of FIG. 5.
Referring to FIG. 5, incident light condensed by microlenses 1 is incident on light receiving surfaces 7, 8, and 9 through color filters 4, 5, and 6 which are provided in openings 2 formed in a mask layer 3. The color filters 4, 5, and 6 are blue, red, and green color filters, respectively. Of light incident on the blue color filter 4, only blue light reaches the light receiving surface 7, because the blue color filter 4 absorbs green and red light. Thus, a light receiving element corresponding to the light receiving surface 7 converts the blue light into an electric signal. Likewise, a light receiving element corresponding to the light receiving surface 8 converts red light into an electric signal, and a light receiving element corresponding to the light receiving surface 9 converts green light into an electric signal.
As is apparent from the foregoing description, taking a case of the light receiving element corresponding to the red light receiving surface 8, for example, green and blue light is not contributable to the photoelectric conversion, so that the photon utilization efficiency decreases. As for the light receiving elements corresponding to the green and blue light receiving surfaces 7 and 9, the photon utilization efficiency decreases in a like manner.
In the conventional color separation unit, as shown in FIG. 5, the blue, red, and green light receiving surfaces 7, 8, and 9 are spatially separate from one another. Thus, when there is the difference in luminance between adjoining colors (pixels), false color, which does not exist in a subject, appears in an image. In other words, when white exists next to black, for example, gray, which does not exist in reality, appears as the false color, because color interpolation is carried out in the prior art by correlating the adjoining light receiving elements. To restrain the appearance of false color, it is necessary to provide false color restraining means such as an optical low-pass filter, an image signal processing, and the like, so that the cost of a device, which is provided with the solid-state image sensor, increases.
Also, there may be cases where the false color restraining means cannot restrain the appearance of false color enough, so that the false color degrades the sharpness of an image.